The present invention relates generally to decoding, and more particularly, to iterative decoding for cascaded low-density parity-check LDPC and trellis-coded modulation TCM coding.
Since coherent optical communication has moved from 100 Gb/s to 400 Gb/s, and even higher using higher-order QAM format, the requirement for high OSNR dramatically shorten the transmission reach. In order to keep the transmission reach within an acceptable distance, coded modulation is one of the promising techniques which could improve the receiver sensitivity by more than 3 dB in sacrificing the spectral efficiency. As a result, the transmission distance is still able to achieve ultra-long-haul distance. The issue with coded modulation is how to integrate with high-coding-gain low-density parity check codes (LDPC) for achieving better error correction performance. This invention record is to propose the concatenation of LDPC and trellis coded modulation (TCM) with iterative decoding feature, to provide high net coding gain (NCG).
The following references discuss prior efforts in the decoding:    [1] Zhou, Wei, et al. “SERIAL CONCATENATION OF TRELLISCODED MODULATION AND AN INNER NON-BINARY LDPC CODE.” U.S. patent application Ser. No. 12/737,107.    [2] Lin, Chien-Ching, and Chen-Yi Lee. “Transmission method combining trellis coded modulation and low-density parity check code and architecture thereof.” U.S. patent application Ser. No. 11/225,081.    [3] Hagenauer, Joachim, and Peter Hoeher. “A Viterbi algorithm with soft-decision outputs and its applications.” Global Telecommunications Conference, 1989, and Exhibition. Communications Technology for the 1990s and Beyond. GLOBECOM'89., IEEE. IEEE, 1989.
In [1], the authors proposed to use TCM as outer codes and non-binary LDPC for inner code. The output of inner LDPC decoder could enhance the performance of TCM with better log-likelihood ratios (LLRs). However, the improvement of the concatenation between non-binary LDPC and TCM is very limited due to the regular LDPC and TCM decoding. The patent did not address the potential bursts errors introduced by the channel. Further, the presence of non-binary LDPC encoder limits the flexibility of this scheme for different transmission systems.
On the other hand, TCM has been used as the inner codes for correcting the errors before being fed into the LDPC decoder in [2]. This approach utilizes the soft-output Viterbi algorithm (SOVA) to decode the TCM encoded symbols, and the output of SOVA can be further used for calculating the LLRs for the following LDPC decoder. However, the performance is also limited by the single concatenation of TCM decoder and LDPC decoder.
Accordingly, there is a need for iterative decoding for cascaded low-density parity-check LDPC and trellis-coded modulation TCM coding that overcomes problems with prior efforts.